Whole-body autoradiography of amouse 5 min after an intrajugular injection of 43 nmoles of l-(4-[<sup>3</sup>H]pyro-Glu)-LH-RH (18.3 Ci/mmole) shows a large accumulation of radio-activity in the pituitary, subcutaneous tissue, intestinal wall, kidney, and bladder. Some radioactivity is also found in the liver, lungs, and heart, but no labeling is seen in the central nervous system. Direct measurements of radioactivity in different organs of the rat 5 min after injection of [<sup>3</sup>H] LH-RH also show that the highest accumulation of radioactivity is in the anterior pituitary gland, kidney, epididymal fat, and skin. Low labeling is measured in the posterior (including intermediate) lobe of the pituitary, pineal, liver, submaxillary gland, testis, adrenal, thyroid, and striated muscle. The pattern of plasma radioactivity after a single intravenous injection of [<sup>3</sup>H] LH-RH can be represented by the sum of four exponents, suggesting a four-compartment model of the disappearance of radioactivity from plasma. The metabolic clearance rate is 1.2 ml/min. The half-life of the first exponent (up to 10 min after injection) is about 7.5 min. 30 sec after injection of [<sup>3</sup>H] LH-RH, the radioactivity is distributed in a total volume of 26.5 ml (approximately 11% of body weight).